EP 0 349 914 B1 discloses a method for the production of tubular nonwoven fabric articles from a strip of a thermoplastic nonwoven support. To form a nonwoven tube, the support is wound up in a spiral form in such a way that the longitudinal edges of the strip overlap along a joining seam. At least in the wound area of the nonwoven tube, this joining seam is formed by thermally welding the longitudinal edges to one another. Winding the strip up into the spiral nonwoven article results in the creation of a hollow rod which, at its free end, has an outlet opening through which a coating solution can be applied to the inside wall of the wound nonwoven tube. By subsequent contact with a coagulation solution, a tubular membrane is obtained.
Tubular membranes of this kind are suitable in particular for ultrafiltration and nanofiltration. These membranes form microporous filter media whose pores are so small that they form a barrier not only to particles of all kinds and shapes, such as pollutants, heavy metals, etc., but also to all microorganisms, for example, in the form of bacteria, parasites and viruses. The active layer of the membranes is also referred to in the technical field as the semipermeable layer. A disadvantage of tubular membranes produced in this way is that, under the pressure exerted by the liquid that is to be filtered, they often tear in the area of the thermal connecting seams, and thus, fail to function properly. To counteract this failure, the German laid-open specification 2 255 989 proposes that the outside periphery of the semipermeable membrane tubes, which are also thin and fragile, should be enclosed by a reinforcement in the form of a braided fiber structure to counter the high hydraulic internal pressure. However, the known production method is complicated, and therefore, expensive. The reinforcement in the form of the braided fiber structure that bears directly on the semipermeable membrane has a disadvantageous effect on the rate of flow through the tubular membrane and on the membrane's filtration properties.
To avoid the problems that arise when the pressure of a liquid causes undesired separation of the thermally produced connecting seams in a spirally wound tubular membrane made from a strip-shaped article, DE 199 09 930 A1 proposes a tubular composite structure composed of a braid of thread bundles and/or wires, preferably in the form of an electron-conducting material and, arranged over this braid, a layer of an ion-conducting material as fuel cell element. In this known solution, aimed among other things at producing what is known as a PEM fuel cell, it is proposed that the tubular inner electrode composed of carbon fibers and/or metal wires be produced by a braiding machine. This tubular braid is centered on a mandrel along which it extends as far as an applicator nozzle for the catalyst coating. The nozzle diameter defines the thickness of the catalyst coat. After a short drying section through ceramic heating elements, for example, the coated braid runs through an annular die via which the ion-conducting membrane is applied in the form of a polymer solution. This step is followed by a longer drying section for expelling the solvent. Thereafter, a second catalyst layer is applied by an applicator nozzle. The outer electrode is then braided around the still pasty catalyst layer. The pasty consistency of the catalyst layer permits penetration of the braid strands and thus permits an intimate connection between catalyst and electrode. If this tubular composite structure composed at least partially of a planar threaded braid is exposed to high pressures from liquids, it is not possible to rule out the possibility of the longitudinal and transverse tensile stresses in the braid causing the threads of the composite structure to shift relative to one another. That shifting can result in undesired stretching effects, particularly in the production of the braid before the actual coating with the membrane material. The consequence of the stretching effects is that the known tubular membrane adopts a shape, and in particular changes in diameter, such that it may be unsuitable for its subsequent intended purpose.